This invention relates to electronic ballasts that provide filament heating before applying the lamp starting voltage, a technique known as programmed starting. More specifically, it relates to ballast circuits that utilize dual-mode inverters to achieve filament preheating.
Fluorescent lamps have filaments that must be heated in order to provide thermionic electron emission. The filaments are coated with an emissive material such as barium oxide that has a low work function. Rapid-start fluorescent lamps have two terminals for each filament so that the filament can be heated with a filament voltage before the lamp is started. Ballasts that supply a filament voltage are commonly known as rapid-start, preheat-start, programmed-start, or soft-start ballasts. The term "soft-starting" is used herein as a generic term for all starting methods that heat the filaments before starting the lamp.
Ballasts that do not supply a lamp filament heating voltage are called instant-start ballasts. These ballasts supply a high starting voltage which causes some of the mercury vapor in the lamp to become ionized. The filaments are heated as they are bombarded by mercury ions which are accelerated by the high starting voltage. An arc is established in the lamp when the filaments become sufficiently hot. Once the lamp has been struck, the arc current flowing through the filaments provides heating, and ion bombardment is greatly reduced. The high level of ion bombardment that occurs during starting causes some of the emissive coating on the filaments to be sputtered away. Lamp failure occurs soon after the emissive material is worn away. Quick-cycle accelerated life tests show that, with properly designed electronic ballasts, typical rapid-start fluorescent lamps can be instant-started 10,000 to 15,000 times before failing.
Lamp life can be improved by using soft starting methods. There are several prior-art methods for achieving soft starting, but each method has significant disadvantages. Many soft-starting ballasts utilize series-resonant inverters. These inverters tend to supply a relatively constant output current, so multi-lamp series-resonant ballasts typically have the lamps connected in series. If one lamp in a series string fails, then all of the lamps are extinguished.
It is desirable to have parallel-connected lamps so that the failure of one lamp will not cause the other lamps supplied by the ballast to extinguish. The term "parallel-connected" means that the ballast has parallel output current paths. Each path includes a lamp in series with a ballasting impedance to limit the lamp current. Current-fed parallel-resonant inverters supply a constant output voltage instead of a constant output current, so they can be used to supply power to parallel-connected lamps.
A current-fed parallel-resonant inverter is a dc-to-ac inverter that has a parallel-resonant tank circuit, a dc choke inductor, and two sets of controlled switching devices that cause an alternating current to flow through the parallel-resonant tank such that an essentially sinusoidal voltage is developed across the parallel-resonant tank. Each controlled switching device set contains at least one controlled switching device such as a transistor. The parallel-resonant tank circuit consists of at least one resonating inductance, and one or more capacitances, each of which is effectively connected in parallel with a resonating inductance. The resonating inductances in the tank circuit are all coupled to each other. They are typically the magnetizing inductances of transformer windings, but current-fed parallel-resonant inverters can also be constructed in a non-isolated manner. Examples of various types of current-fed parallel-resonant inverters are described in U.S. Pat. Nos.: 4,277,726; 4,513,226; 4,692,667; 5,055,746; 5,166,869; and 5,416,386. It should be noted that parallel-loaded series-resonant inverters are sometimes called parallel-resonant inverters, but that is a misnomer. The term "parallel-resonant inverter" as used herein refers specifically to current-fed parallel-resonant inverters. Ballasts that use parallel-resonant inverters are typically referred to as parallel-resonant ballasts, while ballasts that use series-resonant inverters are typically called series-resonant ballasts.
In many soft-starting schemes for both series-resonant and parallel-resonant inverters, the output voltage during the preheating period is great enough that a small lamp current called a glow current is produced. The glow current causes filament erosion due to mercury ion bombardment. Consequently, many soft starting ballasts achieve only a modest improvement over instant start ballasts in the typical number of starts before lamp failure. Some soft starting ballasts have enough glow current that they are actually worse that instant start ballasts. Soft starting ballasts that have a substantial output voltage during the preheating interval are described in U.S. Pat. Nos. 4,277,726 and 5,191,263.
The ideal way to reduce lamp damage during starting is to prevent glow current during the time that the filaments are being heated by maintaining a very low voltage across the lamps during the preheat interval. This technique, known as programmed starting, can substantially increase lamp life in comparison with other methods. Quick-cycle accelerated life tests have shown that properly designed programmed start ballasts can start lamps hundreds of thousands of times before lamp failure occurs. Increasing the number of starts before lamp failure is particularly important when occupancy sensors are used to control ballast operation since they may switch as often as 10 to 50 times per day. In addition to controlling the lamp voltage during starting, programmed start ballasts also control the filament heating voltage. In order to save energy, the filament heating voltage is high during preheating and lower during normal operation.
The ballast output voltage during preheating can be eliminated if an auxiliary inverter is used to heat the filaments. U.S. Pat. Nos. 4,698,553 and 4,928,039 show half-bridge series-resonant ballast circuits that utilize a separate half-bridge inverter to pre-heat the filaments before the main inverter strikes the lamps. After the preheating interval is over, the auxiliary inverter can be shut off to save energy. This approach requires a considerable amount of extra circuitry, and is not suited for parallel lamp operation.
A related approach is shown in U.S. Pat. Nos. 4,700,287 and 4,949,015. These patents show series-resonant ballasts which utilize two half-bridge inverters. During the preheat interval, the first half bridge supplies ac voltage to a filament transformer. During normal operation, the second half-bridge is operated out of phase with the first half-bridge to form a full-bridge inverter which drives a series-resonant tank circuit. As with the auxiliary inverter approach, the half-bridge/full-bridge approach shown in these patents also requires a considerable amount of extra circuitry, and is not suited for parallel lamp operation. This approach further lacks the capability of reducing the filament voltage during normal operation.